Generally, in a mobile communication BS system, a BS amplifies a transmission signal through a high-power amplifier, transmits the amplified signal to an antenna via a feeder cable, and radiates the signal through the antenna. The antenna receives a signal and transmits the received signal to a Low Noise Amplifier (LNA) in the BS through the feeder cable. The LNA amplifies a weak received signal. For the purpose of providing a service, the antenna is mounted on a high place such as a rooftop or a tower and a Base Transceiver Station (BTS) is installed within a building or under a tower. Hence, a long transmission line is established between the BTS and the antenna.
The long signal transmission line between the BTS and the antenna causes a great signal loss during transmission of a transmission signal and a reception signal via the feeder cable. Especially when the distance between the BTS and the antenna is tens of meters, a 3-dB or more loss of an input signal is brought when a link budget is calculated. The signal loss leads to coverage reduction caused by decreased transmit power and the decrease of reception sensitivity caused by a poor reception Noise Figure (NF).
Owing to the recent technological development and cost reduction of transmission power amplifiers, the problem of decreased transmit power can be solved by increasing the output capacity of the power amplifiers. Although the reception sensitivity decrease can be overcome by increasing the output of a Mobile Station (MS), the battery lifetime of the MS may be dropped.
In this context, studies are made on methods for improving the reception NF without imposing constraints on the MS. Among them, a current popular method is that a Tower Mounted Amplifier (TMA) 2 is connected to an antenna 1 in the vicinity of the antenna 1 as shown in FIG. 1, to thereby compensate for NF degradation caused by the loss of the feeder cable. For a related technology, refer to Korean Patent Application No. 2004-16163 entitled “Detachable Tower Mounted Amplifier Directly Connected to Antenna” invented by Deok-Yong Kim, et. al. and filed by the same applicant on Mar. 10, 2005.
The above method has limitations in its effectiveness in overcoming the degradation of a reception NF caused by signal loss in a feeding circuit. Since the TMA 2 amplifies a signal received from each radiation element at one amplifier, a defect in the amplifier causes a rapid degradation in the NF of a received signal as the defective amplifier is usually bypassed. Moreover, a switch for distinguishing transmission from reception in Time Division Duplex (TDD) should have a capacity corresponding to high transmit power.